Callback fn capability + timeout + signalling + better type hinting

Please use >v1.4.0

• One can now define a callback function when registering a command. These can be plain functions or Flask/blinker signals to enable reactive/event-based programming.
• A timeout has been added when invoking commands. This attribute can be passed in the JSON request body. Default value is 3600 seconds.
• Added returncode attribute to response JSON.

Note: Please see Examples section for the relevant code in such use-cases.

• under-the-hood improvements (decreased the memory footprint of the extension to very low value)
• Added type-hinting in almost all places in the source code.

Better Logging, memory management, added result_url to response

• added a result_url field in the POST request's response.
• Much cleaner log messages than before
• Better memory management by reducing some duplicate class instances.
• under-the-hood small bug fixes.

Fixed JSON enc error + faster hashing

• Fixed JSON enc error.
• Faster hashing

Logging + better docs

• Changelog in Pull Request #4.
• new docs: flask-shell2http.readthedocs.io/